I. Field of the Invention
This invention relates to a method for introducing fluxing material into a mold in casting. More particularly the method involves delivering fluxing material into the mold in the manufacture of centrifugally cast steel tubes useful for hydraulic cylinders or similar purposes.
II. Description of the Prior Art
The production of metallic tubes utilizing centrifugal casting molds is well known in the art. These tubes may be pipes or steel tubes used for hydraulic purposes or cast iron pipe, although the casting process usually would include different steps depending on whether steel tubes or cast iron pipes are the desired end product. In the pipe-making process as shown in U.S. Pat. No. 1,949,433, Russell et al, a pouring ladle is generally provided for receiving the molten metal, such as steel or iron, and for accurately pouring a predetermined amount of the molten metal within a predetermined length of time. An inclined trough is positioned to carry the molten metal to the metal mold contained within and rotated by a centrifugal casting machine. The rotating mold within the casting machine is generally surrounded by a water jacket.
Typically the method of casting a metallic tube includes the following steps: first, the ladle and orifice, which may be mounted on a pouring box, are moved to a position whereat the pouring orifice will deposit the molten metal into the mold. Next, the machine ladle is activated whereby it is lifted so the molten metal is discharged into the pouring box. The size of the attached orifice determines the flow rate. The molten metal is discharged along the length of the rotating metal mold, whereby a uniform thickness of the molten metal is deposited upon the interior surface thereof. After the casting has solidified, the tube is extracted from the mold, and the casting cycle, as described above, may be repeated.
The addition of a fluxing material in connection with centrifugal casting techniques is well-known in the prior art and is described, for example, in the patent of Morgan, U.S. Pat. No. 2,265,740, assigned to the assignee of the present invention and disclosing a pipe-making process. The fluxing material is used to form a sealing slag on the inside surface which will contain the impurities that might otherwise be entrapped in the molten metal.
The use of flux also serves to minimize or eliminate lamination defects. A lamination defect results from the sinking of solid oxidized metal films from the inside surface into the wall of the solidifying tube. When solidification occurs on the unsealed inside surface of the tube, the solid metal film is high in oxygen content, because it is exposed to the atmosphere. When this solid metal film sinks into the molten metal, due to its greater density, the deoxidizers in the metal attack the oxygen on the surface of the solidified metal film. The result of this reaction is a plane of inclusions and porosity, which is called the lamination defect. The solidified metal film also traps inclusions which are attempting to float to the inside surface. This is why the solidified film of metal sometimes has two rows of inclusions, one on each side.
By forming a fluid slag which will float on the surface of the molten metal, the flux minimizes oxidation of the molten metal and insulates the molten metal surface, minimizing heat loss to the air. This tends to prevent the formation of a solidified metal film caused by excessive heat loss at the surface of the molten metal. Additionally, the fluid slag which is formed by the flux will contain the impurities which might otherwise be entrapped in the molten metal.
Various techniques for supplying the flux into centrifugal molds have been used in the past. One of these techniques is to supply the flux after all the metal has been poured into the mold. This method is shown, for example, in U.S. Pat. No. 3,863,702 issued to Hallerberg et. al. One of the problems with this technique is that the heat required to melt flux material is extracted from the surface of the molten metal. If enough heat is extracted from the molten metal surface, the surface will cool sufficiently such that a film of metal will solidify thereon, leading to additional lamination problems.
Another prior art technique for introducing flux material is to supply the flux material into the mold just prior to the pouring of the molten metal. A disadvantage of this technique is that the flux material may have a deleterious effect by collecting on the face of the mold.
U.S. Pat. No. 2,265,740, issued to Morgan, describes a technique of supplying a flux material during the pouring of the molten metal into the mold. The molten metal is poured into the slowly rotating mold which serves initially as a trough to distribute the molten metal lengthwise. Upon achieving a lengthwise distribution of the molten metal, the mold is speeded up to distribute a thin layer of molten metal throughout the mold by centrifugal force. Morgan avoids the problem of deleterious contact between the mold surface and flux by delaying the introduction of the flux until this thin layer of molten metal is distributed over all of the interior casting surfaces of the mold. After the mold surface is completely wetted by the molten metal the flux is axially blown into the mold by high energy bursts of air through a nozzle.
Additionally, in the Morgan patent a predetermined initial quantity of fluxing material is placed on the stream of molten metal as it is being poured. This predetermined quantity of fluxing material is used to minimize oxidation at the surface of the molten metal during the pouring period and is small enough to lessen the danger of the fluxing material contacting the mold. In the apparatus used in the Morgan patent, a blast of compressed air is used to distribute the fluxing material into the mold by placing it on the molten metal in the mold. Furthermore, Morgan suggests the use of a rotary blower. However, by blowing the fluxing material into the mold in sudden bursts, surges will occur in the gas lines and the use of such high velocity gases to supply fluxing material may result in a deleterious non-uniform distribution of the fluxing material within the mold.
U.S. Pat. No. 3,303,018, issued to Goss, discloses a method for improving the reaction time between flux and molten metal. The improvement in reaction time between the flux and molten metal in a rotary reactor is accomplished by the use of a series of high pressure gas jets. For example, in FIG. 4 of Goss, a stream of combined molten metal and flux enters the reactor from runner 68, whereat a gas jet emanating from lance 69 impacts the combined stream violently comingling the metal flux. Disadvantageously, the high energy gas breaks up the molten metal, increasing its surface area and exposing it to more oxygen.
U.S. Pat. No. 4,095,643, issued to Farlow et al, and assigned to the assignee of the present invention, discloses an agent feeder for a pipe casting apparatus. That agent feeder uses a relatively low pressure gas to supply chemical agents into the surface of the mold or the surface of the molten metal. Although primarily concerned with the delivery of inoculating or nucleating agents, this patent also suggests the use of the apparatus to supply fluxing material into a molten metal layer formed on the inner surface of a mold.